Actuator unit for a piezo-controlled fuel injection valve

ABSTRACT

An actuator unit suitable for actuating a fuel injection valve of an injection system for internal combustion engines is comprised of a piezoelectric actuator and a hollow body embodied in the form of a spring. Embodying the hollow body according to the present invention can extend the service life of the actuator unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 2004/000565 filedon Mar. 19, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an improved actuator unit comprised of apiezoelectric actuator.

2. Description of the Prior Art

Actuator units of the type with which this invention is concerned areuse among other things in fuel injection systems and in particular infuel injection valves since the switching times of such actuator unitsare very short. The short switching times permit a more exact meteringof the injected fuel quantity and permit an improved shape of theinjection curve over time. In connection with the present invention, thegeneric term “fuel injection valve” is understood to mean all types offuel injection values, for example, the injectors for common railinjection systems or injection nozzles of conventional fuel injectionsystems. A fuel injection valve with a piezoelectric actuator istriggered by applying an electric voltage to the piezoelectric actuator;this causes a rapid expansion of the piezoelectric actuator due to knownphysical effects of piezoelectric ceramic and causes a valve-closuremember to lift away from a valve seat. The piezoelectric actuator has acertain mass that is accelerated in the course of this. If the voltageapplied to the actuator is reduced, this causes the actuator tocontract. As a function of the triggering speed, the inertia of thepreviously accelerated mass of the actuator generates damaging tensileforces in the piezoelectric actuator, in particular causing fractures inthe soldered connections between the individual layers of thepiezoelectric actuator. In order to prevent this kind of damage, it hasbecome common practice to use a cylindrical hollow body in the form of aspring to prestress the piezoelectric actuator in the axial direction. Adevice of this kind is known, for example, from WO 00/08353 (Siemens).This hollow body is bent from a flat metal sheet and welded at the firstjoint thus produced. The first joint extends parallel to thelongitudinal axis of the hollow body.

Among other things, the welding of the first joint has the followingdisadvantages: the welding causes a generally undesirable structuralchange to the hollow body in the immediate vicinity of the welding seam.A second problem is the spatters produced during the welding, which canlead to difficulties in assembly of the actuator unit or can even leadto functional failures of the fuel injection valve when one or morespatters come loose during operation. A third problem is a sinking-in ofthe welding seam (seam sinkage) at the beginning and end of the weldingseam and the resulting notch effect and voltage spikes.

SUMMARY AND ADVANTAGES OF THE INVENTION

The actuator unit according to the present invention has a hollow bodyand a piezoelectric actuator. The hollow body is elastically embodied,prestresses the actuator, is provided with apertures recesses, has ajoint extending parallel to the longitudinal axis, has a bridge piecebetween each pair of recesses, and has a first end and a second end.According to the present invention, the recesses adjacent to the jointare smaller than the rest of the recesses.

Alternatively, it is also possible according to the present inventionfor the bridge piece between a recess adjacent to the joint and anotherrecess adjacent to that recess to be wider than the bridge piecesbetween the rest of the recesses.

A disadvantage in actuator units with hollow bodies whose joints are notclosed is that the spring rigidity in the axial direction is notconstant over the entire circumference. As a rule, the spring rigidityof the hollow body is reduced in the region of the joint. This causesthe hollow body to act on the piezoelectric actuator with forces in theaxial and radial direction in addition to bending moments. This resultsin the uneven exertion of forces and bending moments on thepiezoelectric actuator, which is undesirable.

The embodiments according to the present invention, which can becomprised in embodying the recesses adjacent to the joint as smallerthan the rest of the recesses of the blank and/or embodying the bridgepieces in the region of the joint as wider than in the rest of theblank, serve to intentionally reinforce the hollow body in the regionsin the immediate vicinity of the joint so as to compensate for thereduction in the spring rate in the region of the joint. It is thereforepossible to achieve a spring rate of the hollow body that is constantand/or rotationally symmetrical over its entire circumference so thatthe piezoelectric actuator that the spring force of the hollow body actson is loaded with forces exclusively the axial direction and not withlateral forces or bending moments. This can significantly extend theservice life of actuator units equipped with a hollow body according tothe present invention. It has turned out to be advantageous if the ratioof the width of a bridge piece between a recess adjacent to the jointand a recess adjacent to that recess to the width of the remainingbridge pieces of the blank has a value between 1.3 and 1.9, preferably1.6. This means that the bridge pieces in the immediate vicinity of thejoint are wider, for example by a factor of 1.6, than the rest of thebridge pieces of the blank.

In special cases, it can also be helpful to embody the width of thebridge pieces as a function of the load; the widths of the bridge piecescan differ from one another by up to a factor of 3.

The recesses in the blank are advantageously disposed so that when theblank is formed into a hollow body, they are disposed in planes and theplanes extend parallel to one another. This improves the behavior of thehollow body and makes it easier to manufacture.

It is particularly advantageous if the recesses are disposed in an oddnumber of planes in the axial direction. In exemplary embodiments thatwere tested in practice, 15 or 17 turned out to be an advantageousnumber of planes. Providing an odd number of planes in the blank assuresthat the uppermost and lowermost planes are the same so that thebehavior of the hollow body at its upper end is the same as the behaviorof the hollow body at its lower end. This measure also improves thebehavior of the hollow body in that at its end faces, the hollow bodyonly transmits axially oriented spring forces to the piezoelectricactuator, a booster piston of a hydraulic coupler, or another componentof the injector.

It has also turned out to be advantageous if a number of recesses aredisposed one after another in a plane and this plane forms a right anglewith the longitudinal axis of the hollow body. It is particularlyadvantageous if there is an even number of recesses in a plane. Thisarrangement results in the fact that the spring rate is constant overthe circumference of the hollow body and consequently, no lateral forcesare transmitted to the actuator.

For reasons involving the manufacture and durability of the hollow body,it has turned out to be advantageous if the recesses are embodied asbone-shaped and extend lateral to a longitudinal axis of the hollowbody.

The “bone-shaped” geometry of the recesses can be described in that therecesses are comprised of a middle piece and two head pieces; the headpieces have at least a first radius, the middle piece has a secondradius, and the recesses have a length. Various trials have shownvarious proportions to be favorable among the principle measurements ofthe first radius (R₁), the second radius (R₂), and the length (L), aswell as the width of the bridge piece at the joint in relation to thewidth of the rest of the bridge pieces:

In a favorable embodiment form, the radius R₁ of a recess adjacent tothe joint is smaller by a factor of 0.867 than the radius R₁ of the restof the recesses. In addition, the second radius R₂ of a recess adjacentto the joint is larger by a factor of 1.317 than the radius R₂ of therest of the recesses of the blank. Moreover, the length of a recessadjacent to the joint is shorter by a factor of 0.984 than the length ofthe rest of the recesses. The width of the bridge piece at the joint isexpressed by the equation b>a/2; in particular b=1.4·a/2. A detaileddescription of the related values, in particular the values “a” and “b,”is given below in conjunction with the drawings.

In another exemplary embodiment, it has also turned out to beadvantageous if the recesses adjacent to the joint have the followingdimensions:

-   -   R₁=0.35 mm-0.43 mm, in particular 0.39 mm    -   R₂=4.0 mm-8.9 mm, in particular 5.0 mm to 7.9 mm    -   L=3.5 mm-4.5 mm, in particular 4.0 mm.

In another embodiment form, the recesses adjacent to the joint have thefollowing dimensions:

-   -   R₁=0.41 mm-0.49 mm, in particular 0.45 mm    -   R₂=5.5 mm-6.5 mm, in particular 6.0 mm    -   L=3.7 mm-4.7 mm, in particular 4.2 mm.

For the rest of the recesses that are not adjacent to the joint, thefollowing dimensions have turned out to be favorable:

-   -   R₁=0.43 mm-0.51 mm, in particular 0.47 mm    -   R₂=4.0 mm-4.8 mm, in particular 4.4 mm    -   L=4.5 mm-5.5 mm, in particular 5.0 mm.

In another advantageous exemplary embodiment, the recesses that are notadjacent to the joint have the following dimensions:

-   -   R₁=0.4 mm-0.5 mm, in particular 0.45 mm    -   R₂=5.5 mm-6.5 mm, in particular 6.0 mm    -   L=4.0 mm-4.5 mm, in particular 4.255 mm.

It has also turned out to be advantageous if the first radii of the headpieces of a recess adjacent to the joint are different from each other,which will be explained by way of example below in conjunction with FIG.8 c.

It is also advantageous if the recesses of two adjacent planes areoffset from one another. It is particularly advantageous if the offsetof the recesses of two adjacent planes is equal to half the repeatpattern of the recesses of a plane. The term “repeat pattern” will beexplained in greater detail below in conjunction with FIG. 2. It isparticularly advantageous if the hollow body has a circular crosssection or if the cross section of the hollow body is the shape of aregular polygon.

According to the present invention, the hollow body can also have aregion that is not perforated by recesses at its first end and/or at itssecond end. As a result of this, the spring force transmitted by thehollow body to a cover plate or another component of the injector iscomparatively uniform since the hollow body is intentionally reinforcedin the region of its ends. This translates into a reduction in themaxima of the spring force over the circumference of the hollow body andfurther alleviates the problem of lateral forces introduced into thepiezoelectric actuator by the hollow body.

The hollow body according to the present invention can be used inactuator units in which the piezoelectric actuator is disposed insidethe hollow body and in which the prestressed hollow body acts on thepiezoelectric actuator with compression. This means that the hollow bodyitself is loaded with tension.

The hollow body according to the present invention can, however, also beused in actuator units in which the piezoelectric actuator is disposedoutside the hollow body and the prestressed hollow body acts on thepiezoelectric actuator with compression. In this case, the hollow bodyis usually loaded with compression.

In order to be able to transmit the prestressing force of the hollowbody to the piezoelectric actuator in the best possible way, it isadvisable for the first end of the hollow body to be connected to anupper cover plate or an adjusting disk and for its second end to beconnected to a lower cover plate or a coupler housing. These connectionscan be produced, for example, by means of welding or crimping.

If only a radial fixing of the hollow body is required, then this canoccur by means of an annular groove or a shoulder in the upper and/orlower cover plate or in the adjusting disk and coupler housing. This canbe sufficient, for example, if the hollow body is loaded not withtension but with compression. In these embodiment variants, it isparticularly advantageous that the annular groove and the shouldercenter the hollow body in relation to the piezoelectric actuator or tothe hydraulic coupler. This effect can be further improved if theannular groove and shoulder are dimensioned so that they cause thehollow body to flare slightly during assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will become apparentfrom the description herein below, taken with the drawings, in which:

FIG. 1 shows a first exemplary embodiment of an actuator unit accordingto the present invention,

FIG. 2 shows a second exemplary embodiment of an actuator unit accordingto the present invention,

FIG. 3 shows an example for a blank from which a hollow body is bent,

FIG. 4 shows a perspective view of a first exemplary embodiment of ahollow body,

FIG. 5 shows an exemplary embodiment of a blank from which a hollow bodyaccording to the present invention is bent,

FIG. 6 shows a perspective view of a hollow body that has been bent froma blank according to FIG. 5,

FIG. 7 shows another exemplary embodiment of a blank for manufacturing ahollow body according to the present invention,

FIG. 8 shows another exemplary embodiment of a blank for manufacturing ahollow body according to the present invention,

FIG. 9 depicts the forces that can be transmitted to a hollow bodyaccording to the present invention according to FIG. 8,

FIG. 10 shows another exemplary embodiment of a blank for manufacturinga hollow body according to the present invention,

FIG. 11 depicts the forces that can be transmitted to a hollow bodyaccording to the present invention according to FIG. 10,

FIG. 12 shows another exemplary embodiment of a blank for manufacturinga hollow body according to the present invention, and

FIG. 13 schematically depicts a fuel injection system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first exemplary embodiment of an actuator unit accordingto the present invention. The actuator unit is comprised of apiezoelectric actuator 1, which can be comprised of a number of stackedindividual piezoelectric elements (not shown). The piezoelectricactuator 1 is triggered via contacting pins 2 that are disposedalongside the actuator 1 and are connected to the actuator 1 in anelectrically conductive fashion. The application of a voltage betweenthe contact pins 2 produces a longitudinal expansion of thepiezoelectric actuator 1, which is used, for example, to control aninjection valve in an internal combustion engine. The piezoelectricactuator 1 with the contact pins 2 is disposed inside a hollow body 4embodied in the shape of a tubular spring. The ends of the piezoelectricactuator 1 each rest against a respective cover plate 5, 6, the uppercover plate 6 being provided with feedthroughs 61 through which thecontact pins 2 extend. The upper and lower cover plate 5, 6 are eachconnected to the hollow body 4 in a form-locked manner and/or byfrictional engagement, preferably by means of welding. The welding seamsbetween the upper and lower cover plate 5, 6 and the hollow body 4 arenot shown in FIG. 1. Alternatively, the connection between the hollowbody and the two cover plates 5, 6 can also be produced, for example, bymeans of a crimp, the crimped upper and lower edge regions of the hollowbody 4 each engaging with the cover plates 5, 6 (not shown).

The hollow body 4 and the cover plates 5, 6 act on the piezoelectricactuator 1 with compression by means of a prestressing force. This meansthat before being welded to the upper and lower cover plate 5, 6, thehollow body 4 is prestressed and then welded.

The hollow body 4 is preferably made of spring steel. The hollow body 4is provided with a multitude of apertures, recesses 7 in order to beable to set a desired spring rate with a predetermined wall thickness“s.” For the sake of clarity, not all of the recesses in FIG. 1 areprovided with reference numerals. Since the multitude of recesses 7 canbest be produced by means of punching, the hollow body 4 is as a rulecomprised of sheet metal. First, a blank with the recesses 7 is stampedout of the metal sheet. Then, the blank is bent until it has a circularcross section, for example, or a cross section in the shape of a regularpolygon. This produces a first joint where the two ends of the bentblank meet each other (not shown in FIG. 1).

FIG. 2 shows a second exemplary embodiment of an actuator unit accordingto the present invention, which is integrated into a piezoelectricallyactuated injector 71.

Since the present invention essentially relates to an actuator unit anda hollow body 4 associated with it, not all of the details the injector71 are explained; instead, essentially only the connection of theactuator unit to the injector 71 is described. The remainingfunctionalities of the injector 71 are already known to those skilled inthe art in the field of injection technology and therefore require nofurther explanation.

The injector 71 has a high-pressure connection 73. Highly pressurizedfuel (not shown) is supplied to the injector 71 via the high-pressureconnection 73. If an injection into the combustion chamber, not shown,of an internal combustion engine is to take place, a nozzle needle 75lifts away from its seat, not shown, and unblocks injection orificesthat are also not shown. A piezoelectric actuator 79 actuates a controlvalve 77, which controls the nozzle needle 75. Between the piezoelectricactuator 79 and the control valve 77, a hydraulic coupler 81 isprovided, an enlargement of which is depicted on the right side of FIG.2.

The hydraulic coupler 81 is essentially comprised of a valve piston 83and a booster piston 85 that are guided in a coupler housing 86. Betweenthe valve piston 83 and the booster piston 85, there is a coupler gap87, which is filled with fuel (not shown). This coupler gap 87 isrequired, among other reasons, because the thermal expansioncoefficients of the piezoelectric actuator 79 and the metalliccomponents of the injector 71 differ greatly from each other.

By means of its valve piston 83, the hydraulic coupler 81 actuates thecontrol valve 77, while a projection 89 of the booster piston 85 restsagainst the piezoelectric actuator 79. A hollow body 4 according to thepresent invention, which is prestressed with compression, presses thebooster piston 85 against the piezoelectric actuator 79, thus acting onit with a compressive prestressing force. The first end 15 of the hollowbody 4 rests against a shoulder 91 of the coupler housing 86. The secondend 17 of the hollow body 4 rests against an adjusting disk 93. Theadjusting disk 93 transmits the spring force of the hollow body 4 to theprojection 89 of the booster piston 85 and therefore to thepiezoelectric actuator 79.

In order to position the hollow body 4 concentric to the hydrauliccoupler 81 and therefore also concentric to the piezoelectric actuator79, the diameter D₁ of the shoulder 91 is matched to the inner diameterof the hollow body 4 so that the hollow body 4 flares slightly when itis slid onto the shoulder 91. Since the hollow body 4 according to thepresent invention has a first joint 31 (not shown) extending over theentire length of the hollow body 4, this allows the hollow body 4 toflare relatively easily so that it fits onto the shoulder 91.

If, as in the exemplary embodiment according to FIG. 1, the hollow body4 is acted on with a compressive prestressing force, then it issufficient for it to be supported in the axial direction at its ends 17and 15, as shown in FIG. 1. In order to further improve the radialfixing of the hollow body 4, an annular groove (not shown) canalternatively or additionally be provided in the shoulder 91 and/or inthe adjusting disk 93.

FIG. 3 shows a blank 9 that can be curved to form a hollow body 4according to the present invention. A multitude of recesses 7 arestamped out of the blank 9. For the sake of clarity, reference numeralsare not provided for all of the recesses 7, which are bone-shaped in theexemplary embodiment according to FIG. 2. The blank 9 is rectangular;two opposite edges 11 and 13 of the blank 9 are interrupted by therecesses 7, while the opposite edges 15 and 17 extend in straight lines,uninterrupted by the recesses 7.

The blank 9 is curved or bent to form a cylindrical or polygonal hollowbody so that the edges 15 and 17 constitute the first end 15 and thesecond end 17 of the hollow body 4 (see FIG. 4), i.e. the longitudinalaxis 35 not shown in FIG. 2 (see FIG. 4) of the hollow body 4 extendsparallel to the edges 11 and 13.

When the blank 9 is bent in the above-mentioned fashion to form acylinder or polygon, the edges 11 and 13 touch each other and form afirst joint 31 (see FIGS. 4 and 5), which extends parallel to thelongitudinal axis 35 of the hollow body 4.

The blank 9 contains groups of recesses 7, each of which comprises anumber of recesses in a row. The recesses are separated from one anotherby bridge pieces 19. Here, too, for the sake of clarity, not all of thebridge pieces 19 of the blank 9 have been provided with referencenumerals. When the blank 9 is bent to form a hollow body in the mannerdescribed above, the recesses 7 disposed in a row lie in a plane. By wayof example, a line 20 in FIG. 3 indicates a row of recesses 7 that aredisposed one after another. In the exemplary embodiment of a blank 9shown in FIG. 3, sixteen rows of six recesses 7 are disposed between theedge 15 and the edge 17.

As is clear from FIG. 3, the recesses 7 of two adjacent rows are offsetfrom one another. The offset is selected so that it corresponds to halfof the length of one recess 7 and one bridge piece 19. This measurementfor one recess and two half bridge pieces 19 is indicated by way ofexample in FIG. 3 by the double arrow 21. This measurement is alsoreferred to as the “repeat pattern.” The offset between the recesses 7of two adjacent rows of recesses is labeled with the reference numeral23 in FIG. 3.

When the blank 9 is rolled to form a hollow body 4 (see FIG. 4) and theends of this hollow body 4 are acted on with a compressive force via anupper cover plate 5 (see FIG. 1) and a lower cover plate 6 (see FIG. 1),then the force F acting between the upper cover plate 5 and the edge 15over the circumference of the hollow body 4 has the curve qualitativelydepicted by the line 25 (see FIG. 5). The circumference angle φ beginswith 0° at the edge 13 and ends with 360° at the edge 11.

It is clear that wherever a bridge piece 19 “supports” the edge 15, alarge force F can be transmitted, as indicated by the maxima 27 of theline 25. The sole exception is where the edges 11 and 13 abut eachother. The “cut” recess 7 there, with its parts 7′ and 7″, weakens thestructure of the blank 9 so that the force F transmitted between theupper cover plate 5 and the hollow body 4 is weaker at this point. Thisfact is indicated in FIG. 3 by the maxima 27 of significantly lowervalue for the force F at φ=0° and at φ=360°.

The edge 17 behaves similarly. As is clear from FIG. 3, in the immediatevicinity of the edge 17 at φ=0° and 360°, there is a cut recesscomprised of the parts 7′ and 7″ whereas in the immediate vicinity ofthe edge 15 at φ=0° and 360°, there is a split bridge piece 19 with thehalves 19′ and 19″. This results in a somewhat different force curveover the circumference of the edge 17.

As is clear from the lower F/φ graph in FIG. 3, there are four maxima 27and two local maxima 29 in the vicinity of the edges 11 and 13 at theangles φ=30° and 330° that are significantly lower than the maxima 27.

As a result of this circumferentially uneven transmission of forcebetween the upper cover plate 6 and the edge 15 on the one hand andbetween the lower cover plate 5 and the edge 17 on the other hand, thehollow body 4 produces a bending moment that acts on the upper coverplate 6 and the lower cover plate 5 when the hollow body 4 is attachedwith a prestressing force to the upper and lower cover plates 6, 5. Thisbending moment is naturally also transmitted to the piezoelectricactuator 1, which has an unfavorable effect on its operationalreliability and lifespan. This bending moment is also undesirable inhydraulic valve elements actuated by the actuator unit.

FIG. 4 shows a perspective view of a hollow body 4, which has beenmanufactured from a blank 9 shown in FIG. 3. The rows of recesses 7,which are not individually labeled in FIG. 4, constitute sixteen planesE₁ to E₁₆ that extend perpendicular to the longitudinal axis 35 of thehollow body 4. To illustrate this, one plane E₂ is indicated in FIG. 4.The wall thickness s of the hollow body 4 is also indicated in FIG. 4.

FIG. 5 shows a blank 9 that can be used to manufacture a hollow body 4according to the present invention. It is clear from the full depictionof the blank 9 that a total of seventeen rows of recesses 7 areprovided. When the blank 9 is formed into a hollow body, these seventeenrows constitute seventeen planes in which the recesses 7 are disposed.The edges 11 and 13 constitute the joint 31 in the hollow body. Theedges 17 and 15 constitute a first end and a second end in the finishedhollow body 4. This is why in connection with the finished hollow body4, the reference numeral 17 is used for the first end of the hollow body4 and the reference numeral 15 is used for the second end of the hollowbody 4.

According to the present invention, in the blank 9, the recesses 7 a and7 b adjacent to the edges 11 and 13 have a geometry that has beenaltered in comparison to the rest of the recesses 7, not all of whichhave been provided with reference lines. The different geometries of therecesses 7, 7 a, and 7 b will be explained in greater detail below inconjunction with the detail A from the blank 9. In this exemplaryembodiment, the recesses 7 a and 7 b have the same geometry. As is clearfrom FIG. 4, the recesses 7, 7 a, and 7 b are “bone-shaped.” Each recess7, 7 a, 7 b is comprised of a middle portion 37 and two head portions 39adjoining this. The reference numerals 37 and 39 have been attached byway of example to only a single recess 7. The head portion 39 can bequantitatively described by a first radius R₁ while the middle portion37 can be quantitatively described by a second radius R₂. Anotherimportant geometric value of the recesses 7, 7 a, and 7 b is the lengthL. It has turned out to be advantageous here if the first radius of therecesses 7 a and 7 b is smaller by a factor of 0.867 than the firstradius of the recesses 7. It has also turned out to be advantageous ifthe second radius R₂ (7 a, 7 b) of the recesses 7 a and 7 b is greaterby a factor of 1.317 than the second radius R₂ of the recesses 7 and ifthe length L of the recesses 7 a and 7 b is shorter by a factor of 0.984than the length of the recesses 7.

There is a bridge piece 19 between each pair of recesses 7. The firstrow of recesses 7 that are disposed in the immediate vicinity of theedge 17 is comprised of six recesses 7. The six recesses 7 of the firstrow are disposed so that one recess is split. This recess 7 is dividedinto two symmetrical halves by the edges 11 and 13.

The second row contains four recesses 7 and one each of recesses 7 a and7 b. The recesses 7 a and 7 b are disposed so that they are in theimmediate vicinity of the edges 11 and 13. Since the recesses 7 a and 7b are smaller than the recesses 7, the hollow body 4 is reinforced at acircumference angle φ of 30° and a circumference angle φ of 330°, namelyin those places where the recesses 7 a and 7 b influence the spring rateof the hollow body 4. This reinforcing in the region of thecircumference angles of φ=30° and 330° compensates for the weakening ofthe hollow body 4 by the joint 31 disposed between the edges 11 and 13(see FIG. 4). The result of this measure is clearly shown in the F/φgraph shown above the blank 9. In comparison to FIG. 3, in which thereis a significant drop in the transmittable force in the vicinity of thecircumference angles φ=30° and 330°, in the F/φ graph in FIG. 5, thereare six maxima 27, that all represent the same amount. This means that ahollow body 4 manufactured from the blank 9 according to FIG. 5 has auniform spring rate over the circumference of its ends 15 and 17 so thatthe spring force transmitted by the hollow body 4 to an upper or lowercover plate and/or a shoulder 91 or 93 acts exclusively in the axialdirection and does not exert any lateral forces or bending moments onthe components on which the spring force of the hollow body 4 acts. Ablank 9 according to FIG. 5 can therefore attain the object according tothe present invention.

With regard to the width of the bridge pieces 19, which is labeled with“a” in detail A, and the width “b” of the half bridge pieces 41 betweena recess 7 a and the edge 11 and between a recess 7 b and the edge 17,respectively, the following quantitative relationship has turned out tobe advantageous. The width b of the half bridge piece 41 should begreater than a/2, in particular, should reflect the equation b=1.4·a/2.

FIG. 6 is a perspective view of a detail from a hollow body 4 accordingto the present invention. It is clear from this depiction that therecesses 7 a and 7 b are disposed in the immediate vicinity of the joint31.

FIG. 7 depicts a blank 9 and a detail of the blank 9, which show thedimensions of the recesses 7 and of the entire blank. This blank 9 hasonly recesses 7 and no recesses with the different geometry (7 a and 7b).

FIGS. 8 a, 8 b, and 8 c show blanks 9 or details of blanks 9, with adimensional depiction of the recesses 7 a and 7 b adjacent to the joint31. These exemplary embodiments are also able to attain the objectunderlying the present invention, which is essentially comprised inachieving a prestressing of the piezoelectric actuator 1 and 79 in theaxial direction without exerting any lateral forces.

The embodiment forms whose details are depicted in FIGS. 8 b and 8 chave also turned out to be advantageous. A detailed explanation of thishas been omitted here since the dimensions furnished in theabove-mentioned figures are self-explanatory and the principle design ofsuch a blank 9 has been described in detail in conjunction with FIGS. 3and 5.

However, reference is made to the lower detail B in FIG. 8 c. In it, thefirst radius R₁ of the recesses 7 a and 7 b at the end oriented towardthe edges 11 and 13 (not shown) is composed of three arc segments 43. Inthe middle, there is a first arc segment 43 with a radius of 0.6 mm,which is adjoined at both ends by two second arc segments 45 with aradius of 0.25 mm. The recesses 7 a and 7 b whose geometry is describedin conjunction with FIG. 8 c are exemplary embodiments for recesses inwhich the first radii of the head portions of a recess 7 a or 7 badjacent to the joint differ from each other.

FIG. 9 is an F/φ graph of a hollow body 4 manufactured from a blankaccording to FIG. 8, in various load states. Three lines that correspondto three different forces F1, F2, and F1 depict the load states. It isclear from FIG. 9 that the spring rate of the hollow body 4 is constantover the circumference in a wide range of load states.

FIG. 10 shows another exemplary embodiment of a blank 9 formanufacturing a hollow body 4 according to the present invention. Theblank 9 has the following differences from the blanks described above:

The blank 9 is not perforated in the region of the edges 15 and 17 thatcorrespond to a second end and a first end of the hollow body 4. Thisreinforces the hollow body 4 in the region of its first end 17 and inthe region of its second end 15, which reduces the value of the maxima27 (see FIG. 3, FIG. 5, and FIG. 9).

A second essential measure for improving the hollow body 4 is comprisedin individually adapting the width a of the bridge pieces 19 to theloads that occur. The bridge piece 19.1 in the first row of recesses 7that are disposed in the immediate vicinity of the edges 11 and 13 isthus wider than a bridge piece 19.2 that is disposed in the blank,farther away from the edges 11 and 13. In the exemplary embodimentshown, the width a₁ of the bridge piece 19.1 adjacent to the edges 11and 13 is 1.2 mm, whereas the other bridge pieces 19.2 have a width a₂of only 0.75 mm. Depending of the dimensioning of the bridge piecewidths a₁ and a₂, there can even be an overcompensation for theweakening of the hollow body 4 due to the presence of the joint 31. Thiseffect is demonstrated in FIG. 11, which is an F/φ graph. If the bridgepiece width a₁ is selected as shown in FIG. 10 b, then all six maxima 27are of the same amount. This design is indicated in FIG. 10 b by the“bridge piece width a₁=1.2”. If the bridge piece width in the immediatevicinity of the edges 11 and 13 is further increased, then the springrate of the hollow body at the circumference angles φ=30° and φ=330° isgreater than in the angle regions between them. This results in asuperelevation of the curve in the vicinity of the circumference angles30° and 330°, which is indicated in FIG. 11 by the line “bridge piecewidth 3.”

FIG. 12 shows another exemplary embodiment of a blank 9 according to thepresent invention in which the bridge piece widths have beenindividually determined as a function of the load situation. The blank 9is symmetrical in relation to a symmetry axis 47 so that thedimensioning of the detail A, which depicts a quadrant of the blank 9,represents by reflection the overall dimensions of the entire blank 9(not shown). The reference numerals 7, R₁, R₂, L, 19, 21, and othershave been omitted from FIG. 12 for the sake of clarity. It should alsobe noted with regard to FIG. 12 that the same bridge piece widths areprovided in the first row of recesses and in the 15th row of recesses.In addition, the bridge piece widths are the same in the second, fourth,sixth, eight, tenth, and fourteenth row of recesses. The bridge piecewidths are also the same in the third, fifth, seventh, ninth, eleventh,twelfth, and thirteenth rows of recesses.

In conjunction with FIG. 13, the description below is intended todescribe how the fuel injection valve 116 according to the presentinvention is integrated into a fuel injection system 102 of an internalcombustion engine. The fuel injection system 102 has a fuel tank 104from which an electrical or mechanical fuel pump 108 delivers fuel 106.It feeds the fuel 106 via a low-pressure fuel line 110 to ahigh-pressure fuel pump 111. From the high-pressure fuel pump 111, thefuel 106 travels to a common rail 114 via a high-pressure fuel line 112.A number of fuel injection valves 116 are connected to the common railand inject the fuel 106 directly into combustion chambers 118 of aninternal combustion engine that is not shown.

As a matter of course, each of the characteristics described in thespecification, illustrated in the drawings, or recited in the claims canbe essential to the present invention either individually or incombination with other characteristics.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

1. An actuator unit having an elongated hollow body (4) which iselastically embodied and which prestresses a piezoelectric actuator (1),the hollow body (4) having a plurality of recesses and having a joint(31) extending parallel to its longitudinal axis (35), a bridge piece(19) between each pair of adjacent recesses (7, 7 a, 7 b), the hollowbody (4) having a first end (17) and a second end (15), and the recesses(7 a, 7 b) adjacent to the joint (31) being smaller than the rest of therecesses (7).
 2. An actuator unit comprising an elongated a hollow body(4) having a longitudinal axis (35) and a piezoelectric actuator (1),the hollow body (4) being elastically embodied and prestressing theactuator (1), the hollow body (4) having a plurality of recessesdistributed thereover with bridge pieces between adjacent recesses, andhaving a joint (31) extending parallel to its longitudinal axis (35)with a bridge piece (19) between each pair of adjacent recesses (7, 7 a,7 b), the bridge piece (19.1) between a recess (7 a, 7 b) adjacent tothe joint (31) and another recess (7) adjacent to that recess beingwider than the bridge pieces (19.2) between the rest of the recesses(7).
 3. The actuator unit according to claim 2, wherein the ratio of thewidth of a bridge piece (19.1) between a recess (7 a, 7 b) adjacent tothe joint (31) and another recess (7) adjacent to that recess (7 a, 7 b)to the width of the rest of the bridge pieces (19.2) has a value between1.3 and 1.9, preferably about 1.6.
 4. The actuator unit according toclaim 2, wherein the widths of the bridge pieces (19.1, 19.2) aredimensioned as a function of the load, and wherein the widths (a, a₁,a₂) of the bridge pieces (19) differ from one another by a factor ofabout
 3. 5. The actuator unit according to claim 2, wherein the recesses(7) are disposed in a number of planes (E_(i)) and the planes (E_(i))extend parallel to one another.
 6. The actuator unit according to claim5, wherein there is an odd number (I, where I=11, 13, 15, or 17, forexample) of planes (E_(i)) in which the recesses (7) are disposed. 7.The actuator unit according to claim 6, wherein the recesses (7 a, 7 b)adjacent to the joint (31) have the following dimensions: R₁(7 a, 7b)=0.41 mm to 0.49 mm, R₂(7 a, 7 b)=5.5 mm to 6.5 mm, L (7 a, 7 b)=3.7mm to 4.7 mm.
 8. The actuator unit according to claim 5, wherein anumber of recesses (7) are disposed one after the other in a plane (E₂),and wherein the plane (E₂) intersects the longitudinal axis (35) of thehollow body (4) at a right angle.
 9. The actuator unit according toclaim 8, wherein a plane (E₂) contains an even number of recesses (7).10. The actuator unit according to claim 5, wherein the recesses (7) oftwo adjacent planes (E₁) are offset (23) from one another.
 11. Theactuator unit according to claim 10, wherein the offset (23) of therecesses (7) of two adjacent planes is equal to one half the repeatpattern (21) of the recesses (7) of a plane (E₁).
 12. The actuator unitaccording to claim 2, wherein the recesses (7) are embodied asbone-shaped and extend lateral to a longitudinal axis (35) of the hollowbody (4).
 13. The actuator unit according to claim 12, wherein therecesses (7) are comprised of a middle portion (37) and two headportions (39), the head portions (39) having at least one first radius(R₁), the middle portion (37) having a second radius (R₂), the recesses(7, 7 a, 7 b) having a length (L), and the following equations apply tothe ratios of the recesses (7) disposed in the inner region of the blankto the recesses (7 a, 7 b) disposed adjacent to the joint (31): R₁(7 a,7 b)=0.867×R₁(7) R₂(7 a, 7 b)=1.317×R₂(7) L (7 a, 7 b)=0.984×L (7), andwidth (b) of a halved bridge piece (41) in relation to the joint: b>a/2;in particular b=1.4×a/2.
 14. The actuator unit according to claim 12,wherein the recesses (7 a, 7 b) adjacent to the joint (31) have thefollowing dimensions: R₁(7 a, 7 b)=0.35 mm to 0.43 mm R₂(7 a, 7 b)=4.0mm to 8.9 mm L (7 a, 7 b)=3.5 mm to 4.5 mm.
 15. The actuator unitaccording to claim 12, wherein the recesses (7) have the followingdimensions: R₁(7)=0.43 mm to 0.51 mm, R₂(7)=4.0 mm to 4.8 mm, L (7)=4.5mm to 5.5 mm.
 16. The actuator unit according to claim 12, wherein therecesses (7) have the following dimensions: R₁(7)=0.4 mm to 0.5 mm,R₂(7)=5.5 mm to 6.5 mm, L (7)=4.0 mm to 4.5 mm.
 17. The actuator unitaccording to claim 12, wherein the recesses (7) are comprised of amiddle portion (37) and two head portions (39), the head portions (39)having at least one first radius (R₁), the middle portion (37) having asecond radius (R₂), the recesses (7, 7 a, 7 b) having a length (4), thefirst radii of the head pieces (39) of a recess (7 a, 7 b) adjacent tothe joint (31) are different from each other.
 18. The actuator unitaccording to claim 2, wherein the hollow body (4) has a circular crosssection.
 19. The actuator unit according to claim 2, wherein the crosssection of the hollow body (4) is the shape of a regular polygon. 20.The actuator unit according to claim 2, wherein the hollow body (4) isradially fixed at its first end (17).
 21. The actuator unit according toclaim 20, wherein the first end (17) of the hollow body (4) is radiallyaffixed to an upper cover plate (6) or an adjusting disk (93) by meansof an annular groove (39) or a shoulder.
 22. The actuator unit accordingto claim 21, wherein the first end (17) of the hollow body (4) beingfastened to the upper cover plate (6) by means of welding (41).
 23. Theactuator unit according to claim 2, wherein the hollow body (4) isradially fixed at its second end (15).
 24. The actuator unit accordingto claim 23, wherein the second end (15) of the hollow body (4) isconnected to a lower cover plate (5) or a coupler housing (86).
 25. Theactuator unit according to claim 24, wherein the second end (15) of thehollow body (4) is fastened to the lower cover plate (5) or a couplerhousing (86) by means of welding (41).
 26. The actuator unit accordingto claim 2, wherein the first end (17) and/or the second end (15) of thehollow body (4) has a region that is not perforated by recesses (7, 7 a,7 b).
 27. The actuator unit according to claim 2, wherein thepiezoelectric actuator (1) is disposed inside the hollow body (4) andthe prestressed hollow body (4) acts on the piezoelectric actuator (1)with compression.